By now, drivers should know that auto mpg claims can be exaggerated. If you buy a car that’s listed as getting 47 mpg, you might be satisfied with, say, an average of 45 mpg. But 37 mpg?

In the grand scheme, 37 mpg is pretty decent—except when you purchase a vehicle that promises mileage that’s 10 mpg better. Two Ford hybrids, the Fusion Hybrid SE and the C-Max SE, boast overall ratings of 47 mpg on their windows in new-car dealerships. Yet a new Consumer Reports study indicates that these mileage claims are way overblown.

This is hardly the first test showing that automaker mpg figures are questionable, but there are a couple of things that are particularly surprising about this new study. First is the extent to which the MPG ratings appear to be inflated. In tests, CR found that the Ford Fusion Hybrid SE got 39 mpg overall, which is 8 mpg less than its official EPA rating of 47 mpg. The Ford C-Max SE also boasts an EPA rating of 47 mpg, but the vehicle averaged just 37 mpg in Consumer Reports’ real-life, on-the-road tests.

The second surprising aspect of CR’s announcement is just how common it is for “official” EPA ratings to be higher than the results found in independent testing. This seems to be the case especially when it comes to hybrid vehicles. In previous CR tests, models such as the Toyota Prius, Honda Civic Hybrid, Hyundai Sonata Hybrid, and Honda Insight EX were all shown to get worse gas mileage than what EPA ratings promised. As Consumer Reports points out, it’s the automakers themselves that come up with their own EPA ratings:

It’s worth noting that automakers mostly self-certify their cars. Then, the EPA spot-checks about 15 percent of them with its own tests in a lab.

This may seem like a student grading his own essay, or a food manufacturer rating its own products for nutrition. The idea, though, is that the threat of the EPA’s independent testing keeps automakers mostly honest. Very recently, the system exposed the fact that many Kia and Hyundai models had inflated mpg ratings, which were only brought to light after owners complained and the EPA investigated. The affected models had to have their mpg ratings changed, and folks who own the vehicles with inflated mileage figures are getting small refunds based on how much they drive.

Consumer Reports has passed along its Ford testing data to the EPA, and seeing as the gap in mpg findings is so substantial, it would seem necessary for the EPA to take action with its own tests.

But how could such a sizeable gap come about in the first place? Did Ford just make up the numbers with the hope that they would help sell cars, and that nobody would notice? Not at all, a Ford statement released in response to the study says:

“Early C-MAX Hybrid and Fusion Hybrid customers praise the vehicles and report a range of fuel economy figures, including some reports above 47 mpg. This reinforces the fact that driving styles, driving conditions, and other factors can cause mileage to vary.”

Presumably, because mpg ratings are supposed to reflect average, normal, real-world behavior, neither Consumer Reports’ nor Ford’s tests utilized styles and conditions at either end of the driving spectrum. Jake Fisher, CR’s director of automotive testing, explained to the Detroit Free Press that there is no evidence that Ford was fudging any data on purpose:

Ford may have designed its vehicles to score well on the EPA test and the results are not being duplicated in real life, Fisher said.

Consumer Reports also stated that the Ford hybrid Fusion and C-Max “still deliver excellent fuel economy” even if their mpg figures were adjusted to reflect the new findings. The problem is that even if the fuel economy is good, it doesn’t measure up to driver’s expectations. “Most buyers won’t get anything near 47 mpg in the real world,” says Fisher. “Even though these two Ford hybrids are very efficient, this big discrepancy may leave customers disappointed.”

The main issue here is trust. Ford seems to understand the importance of trust better than anyone. In its just-released list of 13 Trends for 2013, #1 on list is the concept that “Trust Is the New Black.” The Ford press release explained why trust is so important:

Trust as a differentiator: Given its relative scarcity, trust is emerging as a key positive differentiator for brands. Correlation of trust to brand equity has increased by 35 percent since 2009. As a result, brands are having to rethink how they communicate with and reveal themselves to consumers – the more real and authentic they are, the better

I feel your pain. I previously owned a 2001 Ford Escape. I'm not surprised to learn Ford misinformed consumers about their C Max's actual MPG. Ford's Auto service mechanics also mislead their customers. After continuous problems, high expense and bad experience with Ford's Auto Service Shop, I will never buy another Ford. Escaped from the money pit and purchased my new 2013 Toyota Prius with package four earlier this month, November 1st and I'm averaging 59.6 MPG, normal driving. While in ECO mode commuting from work in heavy stop and go traffic, I achieved 70.1 MPG. Impressive!. My gas mileage will continue to improve as I break it in. I want a reliable, fuel efficient, non-plug in, versatile utilitarian hatchback and Prius has established a strong reputation in the marketplace. Best purchase I made this year.

I thought my 2013 C-MAX would be a Prius Killer? NOT! As a returning Ford buyer I feel deceived. I want to support US companies and US jobs. What was Ford thinking when they published 47/ 47/47 estimates? Based on the advertised EPA estimates, I would have been ok with low 40's but 28-33 mpg is not even in the ballpark. This is not an issue about EPA testing standards, but rather an issue about setting false customer expectations in order to promote sales. Ford's "47MPG" marketing campaign tarnished what should have been the roll out of a truly remarkable vehicle, the CMAX. Real world MPG estimates should have been promoted in the mid-30's. No one would have questioned those numbers and the CMAX would have received the accolades it deserves. How these MPG estimates made it through Ford corporate is beyond me! Maybe it was the rush to go to market? I have been accused of not knowing how to drive hybrid. For the record, during the last three years I have leased both a 2010 Prius and 2010 Honda Insight Hybrid, and consider myself an experienced hyper-miler. My mileage in the Prius is 50 plus, the Insight is 40 plus. The C-MAX is a well-built car, with extremely inflated EPA estimates. I respectfully request that this matter be investigated as soon as possible. My efforts to deal with this locally and through Ford customer service have frustrated me to no end. The constant response? "You need to learn to how to drive hybrid type of vehicle ". Is there a difference how I drive Prius Hybrid vs. the CMAX hybrid? I think we all know the answer to that. I need someone at Ford to reach out to me and assist in a proactive manner so we can put this matter to rest.

In Ford’s own brochure for the C-Max on page 4 the mileage is shown as 46.2 avg mpg and on page 16 it’s shown as 21.8 avg mpg. Good job marketing department.;-) Ever heard of Photoshop? Ever thought to push the reset avg. mpg button and coast?

I have a simple Toyota Corolla and the on-board system gives me a mpg of 37.8 mpg for the last 35,000 miles under normal conditions (not just downhill or lots of tailwind). However, my daughters Corolla (exactly the same car but a different color) gets her just 32 mpg. The differences are very often driving habits and how you maintain your car.

The Hyundai Elantra example shows that some manufactures are not trust worthy. I would never believe what Hyundai tells us about mpg of their vehicles.

My 2013 Hyundai Elantra was advertized at 40 mpg highway. EPA claims 38 mpg. On open highway with little or no breaking at 50-60 mph, I get 31 mpg. And I'm not happy about it! So far, complaints to my Hyundai dealer have gotten me nowhere. I'd be open to suggestions as to what my options are.

Why not require manufacturers to collect and disseminate real-world mileage? The ODB-II data from any car taken to a manufacturer affiliated service facility could be used to collect actual mileage information. This could be sorted by location/registration to account for local factors (generally hilly, hot/cold, etc.). It would provide an amazing database and at the very least a real-world average mileage that could be posted.

@PageNotes I like that. I, for one, would happily sign a release that would allow them to collect anonymized data and release it. I know how big data and big corporations work, so don't believe that the data isn't already being collected. Since the Feds mandated that the data be available in a specific format it sure would be nice if they mandated that it be collected and released.

As has been pointed out, driving habits can play a big part in fuel economy. Having the data would allow a comparison of test conditions vs. the way people really drive.